Without Luffy, Zoro would have been executed by the marines in the first couple of chapters.
Also you forget that Kuma works directly for the Revolutionaries. His goal was never to kill Luffy.
Luffy would've found another right hand to get his dream. The whole reason why Luffy is such a powerful figure is because of his ability to attract others to his cause. According to Mihawk that is the more fearsome power for a pirate.
What's Zoro good for? Cutting people.